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Biomedical subjects

J M van Leeuwen

Publications and source records attributed to J M van Leeuwen.

12 recordsLinked to original sources
PubMed2001

Crossover behavior for long reptating polymers.

The Rubinstein-Duke model for polymer reptation is analyzed by means of density matrix renormalization techniques. It is found that the crossover in the scaling behavior of polymer renewal time (or viscosity) arises from the competing effect of the contribution due to tube length fluctuations and higher-order corrections, which are of opposite sign. Experiments which ought to emphasize both contributions are suggested. The exponent describing the subleading scaling behavior of the diffusion coefficient is also investigated.

Journal Article↗
PubMed2001

Elucidation of the pathways of catabolic glutamate conversion in three thermophilic anaerobic bacteria.

The glutamate catabolism of three thermophilic syntrophic anaerobes was compared based on the combined use of [(13)C] glutamate NMR measurements and enzyme activity determinations. In some cases the uptake of intermediates from different pathways was studied. The three organisms, Caloramator coolhaasii, Thermanaerovibrio acidaminovorans and strain TGO, had a different stoichiometry of glutamate conversion and were dependent on the presence of a hydrogen scavenger (Methanobacterium thermoautotrophicum Z245) to a different degree for their growth. C. coolhaasii formed acetate, CO(2), NH(4)(+) and H(2) from glutamate. Acetate was found to be formed through the beta-methylaspartate pathway in pure culture as well as in coculture. T. acidaminovorans converted glutamate to acetate, propionate, CO(2), NH(4)(+) and H(2). Most likely, this organism uses the beta-methylaspartate pathway for acetate formation. Propionate formation occurred through a direct oxidation of glutamate via succinyl-CoA and methylmalonyl-CoA. The metabolism of T. acidaminovorans shifted in favour of propionate formation when grown in coculture with the methanogen, but this did not lead to the use of a different glutamate degradation pathway. Strain TGO, an obligate syntrophic glutamate-degrading organism, formed propionate, traces of succinate, CO(2), NH(4)(+) and H(2). Glutamate was converted to propionate oxidatively via the intermediates succinyl-CoA and methylmalonyl-CoA. A minor part of the succinyl-CoA was converted to succinate and excreted.

Anaerobiosis↗
PubMed2000

Assessment of health risks of large semi-wild herbivores in urbanized areas.

The health risks for both domestic animals and humans caused by large herbivores in self-sustaining ecosystems are largely unknown. The aim of this article is to make an inventory of these risks, to explore ways to manage them in practice, and to make recommendations for the quantification of risks. Potential hazards from herbivores in and around Europe are listed using the data of the OIE (Office International des Epizooties). The desired health status and the implementation of control or surveillance measures are important factors when assessing the risks. Results indicate that a regular yearly system of health monitoring of herbivores is necessary. To get more insight into the importance of certain risks (Infectious Bovine Rhinotracheitis, biodegradation of carrion in the field) epidemiological investigations have to be carried out to assess the risk of transmission in different situations (with or without intervention). Analysing and managing risks enable decision-makers to formulate the conditions for the development of nature reserves. In Europe more has to be done to increase the quality of nature in terms of de-fragmentation and de-isolation, but regulations concerning the health of large herbivores also have to be improved.

Animals↗
PubMed1996

Dynamic symmetry breaking in a model of polymer reptation.

A numerical study of the transition between oriented and nonoriented polymers in a model for reptation is presented. An electric field biases a charged polymer to drift in the direction of its leading end. At low fields this bias is not enough to orient the polymer, but at larger fields it succeeds in orienting the polymer with a definite head and tail. The resulting motion depends on the relative magnitudes of the field and the chain length. For a given chain length, the field plays the role of inverse temperature in this transition. The critical field between these "phases" is found to be a non-trivial power of the length of the chain. This scaling extends over two full decades in the coupling parameter and the simulations performed involved 2 x 10(9) Monte Carlo steps per data point.

Computer Simulation↗
PubMed1988

[Trends in research related to the health of pigs].

The intensification of pig husbandry has important consequences for pig health care. More attention must not only be paid to clinical outbreaks of disease but also to subclinical problems and disturbances of production. More research must be done to improve the efficiency of disease control and beyond this to the optimalization of prevention. The roles of etiology, biotechnology, the research on pathogenesis, epidemiology and immunology in meeting these goals are roughly outlined and illustrated with concrete examples derived from the program of the C.V.I. (Central Veterinary Institute) in the Netherlands. How this research will affect research immediately connected with health care is explored at the end of this article.

Animal Husbandry↗
PubMed1979

[Registering, licensing and investigating vaccines in animal disease control (author's transl)].

The registration, licensing and investigation of veterinary vaccines are discussed with reference to the producer, the user and the authorities. The significance of basic research by the producer, of testing by the authorities, of quality control and of evaluation (laboratory and field studies) in the control of animal diseases are examined more closely. The fact is stressed, that legislation concerning veterinary medicines in general and vaccines in particular should be well defined. All this is regarded as an integral part of recent developments in animal farming in the Netherlands.

Animal Diseases↗
PubMed1978

Quantitation of P metabolism in the rabbit by means of 32P.

Cr2O3 was found to be very effective in correcting the daily variations in the excretion of dry matter, Ca, Mg, and P in rabbit faeces. It can shorten the length of a balance experiment by 50%. We have an idea that a considerable part (about 50%) of the total P retained is incorporated in the organism so quickly that it cannot be observed with the intravenous radioactive tracer method. The excretion of the endogenous faecal P amounted to 20 mg per day. No significant difference was found between the apparent and the actual absorption coefficient of P. The exchangeable P pool of the organism amounted to 690 mg. The experiment results should be subjected to detailed biometric evaluation.

Animals↗